James Ibori, a petty thief who eventually became one of Nigeria's richest men, has received a 13-year jail sentence after admitting fraud of nearly £50m, described by the judge as probably "ludicrously low".

The former governor of Delta state in Nigeria pleaded guilty in February to 10 offences relating to conspiracy to launder funds from the state, substantive counts of money laundering and one count of obtaining money transfer by deception and fraud.

"In the light of other matters, perhaps that is a ludicrously low figure and the figure may be in excess of £200m, it is difficult to tell," Judge Anthony Pitts told Southwark crown court in London on Tuesday. "The confiscation proceedings may shed some further light on the enormity of the sums involved."

The Metropolitan police estimates that Ibori embezzled $250m (£157m) of Nigerian public funds.

Andrew Mitchell, the UK international development secretary, said: "James Ibori's sentence sends a strong and important message to those who seek to use Britain as a refuge for their crimes.

"Corruption is a cancer in developing countries and the [UK] coalition government has a zero-tolerance approach to it. We are committed to rooting out corruption wherever it is undermining development, and will help bring its perpetrators like Ibori to justice and return stolen funds to help the world's poorest."

The Department for International Development (DfID), which funds the Proceeds of Corruption Unit, began investigating Ibori in 2005, alongside Nigeria's anti-corruption unit, the Economic and Financial Crimes Commission.

Robert Palmer, a campaigner with Global Witness, said: "By doing business with Ibori and his associates, these banks facilitated his corrupt behaviour and allowed him to spend diverted state assets on a luxury lifestyle, including a private jet and expensive London houses, while many Nigerians continue to live in poverty."

According to the prosecutor, Sasha Wass, Ibori and his associates used multiple accounts at the banks to launder funds. Millions of pounds passed through these accounts, some of which were used to buy expensive London property.

At Ibori's sentencing hearing, Wass told the court how Ibori – once a "petty thief with his hand in the till" at the DIY store Wickes – amassed a portfolio of six properties outside Nigeria worth £6.9m at a time he was being paid £4,000 a year as state governor of the Delta region.

The properties in London included a flat in St John's Wood, a house in Hampstead, and one in Regent's Park. He also had a £3m mansion in South Africa, and properties in Texas and in Dorset, near to where his children attended private school.

Ibori also bought a fleet of luxury cars, and in three years ran up £920,000 on his American Express Centurion card – a card only available to the super-rich, Wass told the court. In 2005, Ibori instructed a London solicitor to buy a private jet costing $20m.

Global Witness said the case raised serious questions about the due diligence that the banks carried out on Ibori and his associates.

"It's welcome that the FSA [Financial Services Authority] has said it will do more to target banks that fail to tackle financial crime. But unless they issue penalties that really hurt, nothing will change," said Palmer. "And we now need an investigation into how Ibori was able to move so much money through these British banks for so long and whether or not sufficient checks were carried out."

The FSA has said it will take a tougher line in future after an industry-wide review in 2011 of compliance with money-laundering rules. Last month, Coutts, the Queen's banker, was fined £8.75m for breaches of money-laundering rules after three years of "serious" and "systemic" problems in handling the affairs of customers vulnerable to corruption because of their political links.

Ibori, 49, changed his original plea as his trial was about to begin and admitted stealing money from Delta state and laundering it in London through a number of offshore companies. Ibori admitted to fraud totalling more than $79m, said to be part of total embezzlement that could exceed $250m.

As governor of Delta state, he racked up credit card bills of $200,000 a month and owned a fleet of armoured Range Rovers, said the prosecution. He was trying to buy a plane at the time he was arrested. Once seen as one of Nigeria's wealthiest and most influential politicians, Ibori was seized in 2010 in Dubai at the request of the Metropolitan police and extradited to London last year.

DfID said Ibori systematically stole public funds, depositing them in bank accounts across the world. A major breakthrough came when officers from Scotland Yard found two computer hard drives that revealed the extent of his crimes.

In January, Andrew Mitchell issued an unreserved apology to an Anglo-Nigerian whistleblower after the BBC's Newsnight revealed DfID had betrayed his identity to the people he was accusing. Dotun Oloko had warned DfID about potential problems with CDC investments. The department passed a document containing Oloko's name to ECP, which he had accused of investing in potential corruption.